The use of liquid natural gas (LNG) as an alternative energy source for powering vehicles and the like is becoming more and more common as it is domestically available, environmentally safe and plentiful (as compared to oil). As a result, the need for dispensing systems and methods that accurately meter cryogenic liquids, such as LNG, has grown.
An example of an effective prior art cryogenic liquid metering system is provided in commonly assigned U.S. Pat. No. 5,616,838 to Preston et al., the contents of which are hereby incorporated by reference. The '838 patent discloses mounting a cryogenic liquid meter within an insulated cryogenic metering container so that the meter is submerged in cryogenic liquid that is provided from a storage tank and dispensed. This avoids two-phase flow through the meter and permits accurate metering without the need to initially circulate the cryogenic liquid through the meter to pre-cool the meter (prior to each dispensing session).
In addition, the '838 patent discloses that a volumetric flow rate of the cryogenic liquid being dispensed is read by the meter, and that this data is provided to a microprocessor. Temperature data from a temperature sensor positioned in the cryogenic metering container, or differential pressure data from a pair of vertically spaced pressure sensors positioned in the cryogenic metering container, is provided to the microprocessor so that the density of the cryogenic liquid being dispensed may also be determined. The microprocessor is then able to calculate and display the metered amount of cryogenic liquid/LNG dispensed to the use device.
While the system of the '838 patent performs well, the unknown composition of most LNG makes use of temperature to determine density (i.e. “temperature compensation”) unacceptable. LNG is made up of mostly methane, but includes different levels of hydrocarbons, such as carbon dioxide and nitrogen.
With regard to use of pressure differential data to determine density, the dynamic nature of the cryogenic liquid as it flows into and out of the cryogenic metering container creates issues such as “noise” in the taps of the pressure sensors.
A need therefore exists for a metering system and method for cryogenic liquids that addresses at least some of the above issues.